The present invention relates to a cathode ray tube used for a reflection type projection display or television system such as a video projector.
U.S. Pat. No. 4,024,579 discloses a color projection television receiver and projector having three color tubes mounted in fixed relation and in predetermined location relative to a screen for projecting three color pictures in registration on the screen. Accurate registration of the pictures results from a projection tube structure adapted for this purpose, whereby field adjustment is minimized. The tube structure employs a mounting barrel accurately suspended within the evacuated envelope and supporting a phosphor-coated target and a projection mirror. A Schmidt correction lens is mounted externally of the envelope of each tube. There is room for improvement in such color tubes as will be described below.
Similar considerations apply to British Patent Publication No. 2,013,974 disclosing a projection television tube with a spherically curved phosphor screen and mirror mounted concentrically at the end of a cylindrical envelope wall by means of a concentrically shaped sealing edge. The screen may be mounted separately from the face plate, which may be plane.
A conventional projection tube as shown in FIG. 1 comprises a face plate 1 of the concentric meniscus type, a target 2 consisting of a metal backing 3 and a luminescent composition 4 disposed on the inner center portion of the face plate 1, an end plate 5 having a spherically concave inner surface, and a mirror 6 made by metal evaporation onto the inner surface of the end plate 5. From the center portion of the end plate 5, a neck 7 is extended outwardly along the longitudinal axis of the tube. The face plate 1 and the end plate 5 are sealed by means of a cylindrical member 8 having a given length so as to form an envelope 11. The inner surfaces of both plates 1, 5 and the axially facing edge surfaces of the housing member 8 are polished to align these parts relative to the same center of curvature, and are sealed with frit glass 9, 10 at each end of the housing member 8.
An electron gun 12 and an electron beam focusing coil 15 are affixed to the neck 7. A correction lens 14 for the spherical abberation caused by the mirror 6 is attached to a supporting frame 13 held in position in front of the face plate 1. The lens 14 such as of circular clear plastic lens serves to correct the spherical aberration, but a concentric meniscus lens may be used as a spherical aberration correcting means. A projection screen 16 is located in front of the lens 14.
In the operation of the prior art tube of FIG. 1, an electron beam 17 produced by the electron gun 12 is directed toward the target 2 to build an image thereon by scanning the electron beam. A light ray 18 of the image is reflected by the mirror 6, and a reflected ray 19 of light is passed through the face plate 1 and correction lens 14 so as to focus an enlarged image on the screen 16 set in the forward position of the tube.
As shown in FIG. 2, when a beam spot "t" of the image is formed at the target 2 on an optical axial line 20 produced between the spot "t" and a center O of curvature of the mirror 6, the ray 18.sub.1 of light from the beam sport "t" is reflected on the mirror 6 at a point m.sub.2 near to a point m.sub.1 positioned on the optical axial line t-O. The reflected ray 19.sub.1 of light advances symmetrically in a direction defined by an angle .alpha. to a normal line 21.sub.1 of the mirror surface and focuses at a focusing point f.sub.1 formed by a crossing point between the line of the reflected light ray 19.sub.1 and the optical axial line 20 extended through points m.sub.1, t and O. Similar to the above, another ray 18.sub.2 of light from the beam spot "t" is reflected at a point m.sub.3 spaced from the point m.sub.1 on the mirror 6, and the reflected light ray 19.sub.2 advances symmetrically in a direction defined by an angle .beta. to a normal line 21.sub.1 of the mirror surface to focus at a focusing point f.sub.2 formed at the crossing point between the line of the reflected ray 19.sub.2 and the optical axial line 20.
Thus, many reflected rays of light caused by the beam spot "t" focus in every point within the fixed range along the optical axial line 20, and the focused images of the beam spot "t" become quite indistinct. However, arrangements to make the images distinct are achieved by the alignment of a spherical aberration correction lens 14 such as a Schmidt's lens as shown in FIG. 2 at the center O of curvature of the mirror 6 so as to refract reflected light rays 19.sub.1, 19.sub.2 at each point m.sub.2, m.sub.3 . . . on the mirror surface for focusing each reflected light ray into an image at a point "s" on the screen 16.
In this connection, the light ray reflected at the periphery of the mirror 6 has a large reflection angle compared with those of the center of the mirror. Thus, the spherical aberration becomes large. It was sometimes difficult to obtain a sufficiently distinct image or a good image resolution even by the use of the correction lens 14, when the spherical aberration becomes very large. This is also true if a slight distortion of sphericity of the mirror 6 occurs. In order to improve the image resolution, it has been proposed that at the top of the cylindrical supporting frame 13, a flange 22 extending inwardly and forming a diaphragm is provided to mask the periphery of the correction lens 14 so as to shield some of the rays which are reflected at the periphery of the mirror 6 with a large spherical aberration, please see FIGS. 1 and 3.
If the periphery of the correction lens 14 is masked by the flange 22, total light quantity passing through the correction lens 14 is, however, reduced. The resulting brightness decrease of the projection image onto the screen 16 causes considerable problems. The effective area for transmitting light may be calculated as follows. The diameter of the correction lens 14 is 2r.sub.1. The radial breadth of the flange 22 is "w". Thus, the effective light transmitting area S.sub.1 of the correction lens 14 is represented by S.sub.1 =.pi.r.sub.1.sup.2 when the flange 22 is not used. When the flange 22 is attached the effective area is S.sub.2 =.pi.(r.sub.1 -w).sup.2. Thus, the effective area S.sub.2 /S.sub.1 is expressed by S.sub.2 /S.sub.1 =(r.sub.1 -w).sup.2 /r.sub.1.sup.2 =(l-w/r.sub.1).sup.2 =l-2w/r.sub.1. This means that the use of the flange 22 decrease the transmitted light quantity by about 2w/r.sub.1. Assuming that the diameter 2r.sub.1 is 150 mm, and the breadth "w" is 10 mm, it follows that the brightness of the projection image is decreased by 2w/r.sub.1 =27%.
In addition, even if the target 2 formed on the face plate 1 and the mirror 6 formed on the end plate 5 are polished into perfect spherical surfaces, it has happened some times that the polished surfaces of the face plate 1 and of the end plate 5 will be slightly deformed when they are heated up to a high temperature in the frit sealing step for securing the plates 1 and 5 to the envelope 11. The deformation of the whole surface occurs seldom, but most of the deformations take place partially. It is noted that the mirror 6 tends to deform at the periphery rather than at the center so that the problem of the proper image resolution at the edge portion of the screen 16 becomes important.
In other words, the spherical aberration of the light ray 18 reflected at the periphery of the mirror 6, is larger than the aberration at the center of the mirror 6. Therefore, a larger degree of correction must be applied to the reflected ray 19 at the periphery as compared to that at the center. However, there is a certain limit for correcting the aberration due to the lower resolution of the image at the circumference of the screen 16. It may be proposed to use the wide flange 22 as an integral part of the supporting frame 13 as described above. The wide flange 22 covers the periphery of the correction lens 14 to shield the reflected ray of light passing through the peripheral portion of the correction lens 14, whereby this arrangement may contribute to improving the resolution of the image projected on the screen 16. However, another problem arises due to decreasing the image brightness on the screen because of the reduction of the effective aperture by covering the whole periphery of the correction lens 14 with the wide flange 22.
In a color projection television system a series of projection tubes B, G and R are arranged in front of the screen 16 to get blue, green and red color components as shown in FIG. 4. The projection tubes B and R for the blue and red color components are located to the right and left of the tube G and hence have different distances relative to the screen 16. Accordingly, the resolution will be reduced to different degrees by each of the tubes B and R at specific circumferential side edge portions of the screen 16.